Virginiamycins S and M belong to a family of antibiotics generally known under designations such as "synergistins", "synergimycins" and "streptogramins". These antibiotics, as produced by streptomycetes, are mixtures of two types of compounds, A and B, which in vivo display a synergistic activity. Type A compounds from different microorganisms are similar as well as the type B compounds. However, the structures of A and B compounds are different.
Numerous work on these antibiotics have been published and several products are available under different commercial names (cf. the Merck Index, 10th ed., 1983, Merck & Co, Inc., Rahway, N.J., US, pag. 1432, Nx9810 "Virginiamycin").
Techniques for production of antigenic modification of polypeptides capable of inducing the formation of antibodies are also known (cf., e.g., EP, A, 0117934, The Ohio State University, Sep. 12, 1984, pag. 1-7).